Field of the Invention
The present invention relates to an arrangement for generating a zero-current pulse for generating a zero-current crossing in an electrical component through which a direct current flows, in particular a vacuum interrupter.
Description of the Related Art
A vacuum interrupter is frequently employed as a load or power switch for currents in alternating current networks. To switch off the anode current or the switched current, the vacuum interrupter here requires a negative voltage which is provided by the negative half wave of the alternating voltage. In the case in which a direct current is to be interrupted, a current pulse, or a zero-current pulse, which can be superimposed onto the direct current in order to generate the necessary zero-current crossing, is required, as a result of the absence of a zero crossing.
In the method known until now for generating an artificial zero-current crossing by means of a zero-current pulse, a simple resonant RLC circuit (a resonant circuit based on resistor, inductor, capacitor) is usually employed. If the direct current is to be switched off, the vacuum interrupter is opened, the zero-current pulse is impressed, and the current interrupted. A zero-current pulse generated by a resonant RLC circuit here has a sinusoidal current curve. The value of the frequency of the resonant RLC circuit normally here lies in the range of kilohertz, and is thus significantly above the frequencies that typically occur in alternating current networks.
The interruption of current by the vacuum interrupter occurs relatively reliably up to a certain maximum current gradient dI/dt (time derivative of the current) at the zero-current crossing. The current gradient of the resonant RLC circuit here corresponds to a cosine function. The dimensioning of the resonant RLC circuit can only be optimized for the level of a particular, specifiable current. With different switched currents, and a zero-current pulse that remains the same, different current gradients, which are not necessarily optimum, therefore emerge at the zero crossing of the switched current at the time when the current is interrupted.
A resonant RLC circuit that is designed to generate a Zero-current pulse with a high amplitude thus exhibits an initially very high current gradient which however falls according to the cosine function with increasing time and amplitude. If the direct current that must be compensated for is large, the zero-current crossing thus occurs at a time at which the current gradient has already fallen in accordance with the cosine function, and is thus sufficiently low. If, however, the direct current that is to be compensated for is low, a zero-current crossing already occurs at an early point in time at which the current gradient of the zero-current pulse is still very high, possibly being too high.